Glass-Ceramic Plate and Method for the Production Thereof

ABSTRACT

The present invention relates to a glass-ceramic plate intended for example to cover or receive at least one heating element, in particular intended to serve as a cooking plate, said plate having a coating comprising at least two contrasted zones. 
     The invention also relates to the method and device for producing the plate and cooking apparatus including said plate.

The present invention relates to a glass-ceramic plate intended inparticular to cover or receive heating elements, in particular intendedto serve as a cooking plate, the heating elements associated with thisplate being in particular of induction heating hobs.

Sales of glass-ceramic cooking plates have been constantly increasingfor several years. This success is explained in particular by theattractive appearance of these plates and by their ease of cleaning.

It should be recalled that a glass-ceramic originates from a glass,called a precursor glass, of which the specific chemical compositionmakes it possible to bring about controlled crystallisation by suitableheat treatments called ceramization. This specific partly crystallizedstructure gives the glass-ceramic unique properties.

Various types of glass-ceramic plates currently exist, each variantbeing the result of sizeable studies and many tests, given that it isvery difficult to make modifications to these plates and/or to themethod for obtaining them without risking an unfavorable effect on thedesired properties: in order to be usable as a cooking plate, aglass-ceramic plate must in particular have a transmission inwavelengths of the visible region that is at the same time sufficientlylow in order to mask at least partly the subjacent heating elements atrest and sufficiently high, so that, with the aim of safety, the usercan visually detect the heating elements in the operating state.Generally, it must also have a high transmission at wavelengths in theinfrared region.

The first plates developed have therefore been plates with a dark color,in particular black. More recently, other plates have been developedwith a lighter appearance (in particular white) and having for example ablur or haze of at least 50% (as described in patent FR2766816), thepreviously mentioned criteria (such as the detection of heating elementsin the operating state without dazzle from said elements or without anunattractive appearance of these) sometimes being taken into account tothe detriment of other aspects (such as the clear appearance of anydisplay positioned under the plate).

The present invention has sought to develop a novel type ofglass-ceramic plate making it possible to widen the range of existingproducts, in particular an improved glass-ceramic plate having a morefunctional novel appearance, without the modifications provided havingharmful effects on the normally desired properties.

The novel plate according to the invention is a glass-ceramic plate,intended for example to cover or receive at least one heating element,in particular intended to serve as a cooking plate, said plate having acoating comprising at least two contrasted zones.

Advantageously, the plate is based on a transparent or translucentglass-ceramic and the coating is intended to mask, at least partly, thesubjacent elements at rest (such as the heating elements and anydisplays), while enabling the heating elements and any displays to bedetected when they are in service. The aforementioned zones (or regions)are chosen so as to meet the objectives mentioned, and in particularexhibit a contrast (or opposition highlighted by their juxtaposition)with this aim that can be expressed in different ways (juxtaposition ofmatt/gloss effects, differences in thickness etc.) as explainedhereinafter.

In general, one of the aforementioned zones (or group of zones as statedhereinafter) constitutes the zone, referred to as a background zone,occupying the majority of the surface area of the coating (generally 50to 99%) and preferentially intended to mask the main part of the heatingstructure that the plate must cover, and the other aforementioned zone(or the other group of zones contrasted in relation to the backgroundzone) is preferentially a zone referred to as a display zone, forrevealing, in relation to the background, a decoration and/orinformation (trademark, logo, symbol etc.) and/or the location offunctional elements such as heating elements, control devices, displaysetc., this zone also making it possible to detect whether or not theseelements are in an operational state (the surface area covered by thelatter zone generally representing 1 to 50% of the surface area of thecoating). From now on, a zone (according to the number of zones of eachcategory present on the plate) will be understood to mean a singlewell-defined zone (when this zone is unique in its category) as well asa group or assembly of zones identically coated or having the sameeffect (in relation to the desired contrast between the zones or groupsof zones) when there are several defined zones of the same category.

The glass-ceramic base or bare substrate forming the plate (before theaddition of the coating) is preferentially transparent or translucent aspreviously indicated, and generally has a light transmission T_(L) (inthe visible wavelength region—integrated between 0.38 μm and 0.78μm—according to the illuminant D₆₅) greater than 50%, in particularbetween 50 and 90%. The coating (and the layers constituting it assubsequently explained) is preferentially chosen so that the plateprovided with the coating according to the invention has, at thelocation of its background zone, a very low light transmission, (inparticular less than 5 or 10%, or even nearly zero or zero), the displayzone being able as for it to have a light transmission for example lessthan 20% (in particular between 0.2 and 20%), in particular below 10%(in particular between 0.5 and 10%).

Advantageously, the coating with contrasted zones is principally (andpreferably exclusively) situated on only one and the same face of theplate, advantageously the face intended to be turned towards (orpositioned facing) the heating element or elements in the position ofuse (face generally referred to as the lower or inner face).

Preferably, according to the invention, the two contrasted zones are atleast one zone with a shiny or reflecting or mirror effect, hereinaftercalled a reflecting zone, and at least one zone with (in particularcomparatively) a matt effect (having in particular an at least partialopacifying or masking effect) hereinafter called the matt zone. Inanother also advantageous embodiment, the two contrasted zones are zoneswith coatings with variable thicknesses, the thickest zone substitutingitself for example for the aforementioned matt zone and the thinnestzone substituting itself for the aforementioned reflecting zone.

Preferably, the matt zone (or thicker zone) is the zone called thebackground zone and the reflecting zone (or the thinner zone)preferentially constitutes the zone called the display zone.

The plate according to the invention has many advantages, technical aswell as aesthetic, as will also appear in the remainder of thedescription. In particular, it makes it possible to mask the heatingelements, control devices and associated displays when they are not inservice, and enables them to be detected visually by the user when theyare in service without the user being dazzled. If need be, thereflecting zone permitting at the same time this masking at rest and thedetection of the elements in service, also has the advantage of makingit possible to use indicators (of duration, power, etc.) such aslight-emitting diodes, of various colors, contrary to the usual opaqueor dark colored plates that only permit the use of indicators emittingin the red region.

The plates according to the invention are practical in use and easy toclean. The positioning of all the coating on the lower face makes itpossible to prevent, if need by, unattractive optical effects such as asplit image, at the same time as it makes it possible to preventproblems of the abrasion of said coating when the exposed face (outer orupper surface turned towards the user) of the plate in the use positionis cleaned. The constituents of the plate are also chosen so that it hasgood mechanical strength and heat resistance and is compatible with theuse of subjacent heating elements such as induction electric heatingelements, or possibly other types of elements such as atmospheric gasburners.

A glass-ceramic plate is hereinafter understood to mean not only platesmade of glass-ceramic proper but also plates in any other similarmaterial resistant to a high temperature and having a coefficient ofexpansion that is zero or nearly zero (for example less than15×10⁻⁷K⁻¹). Preferably however it consists of a plate made ofglass-ceramic proper.

The coating is preferentially formed of at least two layers positionedso as to form the aforementioned zones, one of the layers beingpreferentially a layer referred to as the paint layer (generally basedon a resin or resins), resistant to high temperatures, said layergenerally covering only the background zone, the other layer beingpreferentially a reflecting layer or a thinner layer of paint than thepreceding one and generally covering at least the display zone.

The layer or layers of paint used are advantageously chosen so as toresist high temperatures and to have stability in time as regards theircolor and cohesion with the plate and so that they do not affect themechanical properties of the plate.

The paint or paints used advantageously have a degradation temperaturegreater than 350° C., in particular between 350° C. and 500° C. They aregenerally based on a resin or resins, where appropriate filled (forexample with pigment(s) or dye(s) and are, according to requirements,diluted in order to adjust their viscosity with a view to theirapplication onto the glass-ceramic, the diluent or solvent (for examplewhite spirit, toluene, etc.) being as the case may be eliminated duringsubsequent baking of the paint or paints.

For example, the paint may be a paint based on at least one siliconeresin, in particular a silicone resin modified by the incorporation ofat least one alkyd resin. Pigments may also be added as colorants, suchas pigments for enamels (chosen for example from compounds containingmetal oxides, such as chromium oxides, copper oxides, iron oxides,cobalt oxides, nickel oxides, or from copper chromates, cobaltchromates, etc.), TiO₂ etc.

In a particularly preferred manner, the paint used includes (or is basedon) at least one high temperature resistant (co)polymer (in particularhaving a degradation temperature greater than 400° C.), it beingpossible for this paint to include or not to include at least onemineral filler so as to ensure its cohesion or its mechanicalreinforcement and/or its coloration. This (co)polymer or resin may inparticular be one or more of the following resins—polyimide resin,polyamide resin, polyfluorinated resin and/or polysiloxane resin.

Polysiloxane resins are particularly preferred: they are colorless andtherefore capable of being colored, (for example with fillers orpigments giving them the desired color); they may be used in thecrosslinkable state (generally by the presence of SiOH and/or SiOMegroups in their formula, these groups participating more often in anamount of 1 to 6% by mass of their total mass) or they may be converted(crosslinked or pyrolyzed). They advantageously have, in their formula,phenyl, ethyl, propyl and/or vinyl units, very advantageously phenyland/or methyl units. They are preferably chosen frompolydimethylsiloxanes, polydiphenylsiloxanes, phenylmethylsiloxanepolymers and dimethylsiloxane/diphenylsiloxane copolymers.

The crosslinkable polysiloxane resins preferentially used generally havea weight-average molecular weight (M_(W)) of between 2000 and 300,000daltons.

In a nonlimiting manner, it may be indicated that Dow Corning® resins804, 805, 806, 808, 840, 249, 409 HS, and 418 HS, Rhodorsil® 6405 and6406 from Rhodia, Triplus® from General Electric Silicone and SILRES®604 from Wacker Chemie GmbH, used alone or mixed are perfectly suitable.

The resins chosen in this way are in particular able to withstandinduction heating and may also be suitable (in particular the abovepolysiloxane resins) for other types of heating (gas burner or evenradiant or halogen heating).

The paint may be free from mineral fillers, in particular if itsthickness remains low. However, such mineral fillers are generally usedfor example to reinforce mechanically the deposited paint layer, tocontribute to the cohesion of said layer, to contribute to itsattachment to the plate, and to counteract the appearance andpropagation of cracks within it etc. For such purposes, at least afraction of said mineral fillers preferentially have a lamellarstructure. The fillers may also take part in coloration. Whereappropriate, several types of complementary fillers may participate (forexample colorless fillers for mechanical reinforcement and other fillerssuch as pigments for coloration) The effective quantity of mineralfillers generally corresponds to a volume rate of 10 to 60%, moreparticularly 15 to 30% (volume rates based on the total volume offillers and paint).

The thickness of each paint layer deposited may lie between 1 and 100microns, in particular between 5 and 50 microns. Application of thepaint or resin may be carried out by any suitable technique, such asdeposition by brush, blade, spraying, electrostatic deposition, dipping,curtain coating, deposition by screen-printing etc., and is preferablycarried out by screen-printing (or possibly deposition with a blade).

Deposition may also be followed by a heat treatment intended to ensure,according to the case, drying, crosslinking, pyrolysis etc. of the layeror layers deposited.

Preferably, the coating according to the invention comprises at leastone layer of paint in which the resin has been, at least partly,crosslinked and/or pyrolyzed, partially or completely, and/or has notbeen heat treated (the resin may optionally be intended to be eliminatedin places where it has not been heat treated) said paint layerconsisting, partly or completely, of a mixture a) of mineral fillers andb) of at least one crosslinkable polysiloxane resin (nearly) free fromcarbonaceous material(s) precursor(s) and/or of at least one crosslinkedpolysiloxane resin (nearly) free from carbonaceous material(s) andcarbonaceous material(s) precursor(s) and/or of a porous inorganicmatrix based on silica (the resin having been for example pyrolyzed andtherefore mineralized), (nearly) free from carbonaceous material(s), themineral fillers being distributed in the resin or matrix.

As regards the reflecting layer or layers used as the case may be,according to the present invention, these layers may for example belayers with a mirror or shiny effect, based on thin layers or stacks ofthin layers or including effect pigments, etc.

According to a first particularly advantageous embodiment, at least onereflecting layer is used formed of at least one (sub)layer of themetallic type and/or at least one (sub)layer based on a dielectric-basedmaterial, this reflecting layer generally and advantageously having amirror effect.

This layer may therefore for example be at least one single metallic orsubstantially metallic layer (for example a thin layer of Ag, W, Ta, Mo,Ti, Al, Cr, Ni, Zn, Fe or an alloy based on several of these metals, ora thin layer based on stainless steels etc.) or may be a stack of(sub)layers comprising one or more metallic layers, for example ametallic (or substantially metallic) layer advantageously protected(coated on at least one face and preferably on its two opposite faces)by at least one layer based on a dielectric material (for example atleast one layer made of silver or aluminum coated with at least oneprotective layer made of Si₃N₄— in particular anSi₃N₄/metal/Si₃N₄—stack—or made of SiO₂).

It may alternatively consist of a monolayer coating based on adielectric material with a high refractive index n, namely greater than1.8, preferably greater than 1.95, and in a particularly preferredmanner greater than 2, for example a monolayer of TiO₂ or of Si₃N₄, orof TiN, or of SnO₂ etc.

In another advantageous embodiment, the reflecting layer is formed of astack of thin (sub)layers based on dielectric material(s) withalternately high refractive indices (preferably greater than 1.8, oreven 1.95, or even 2 as previously explained) and low (preferably lessthan 1.65) refractive indices, in particular of a material or materialsof the metal oxide (or metal oxynitride or nitride) type, such as TiO₂,SiO₂ or a mixed oxide (tin-zinc, zinc-titanium, silicon-titanium etc.)or an alloy etc., the (sub)layer deposited as the case may be first ofall and being therefore located against the inner face of the plate,being a layer with a high refractive index.

As a (sub)layer material with a high refractive index, mention may bemade for example of TiO₂ or possibly SnO₂, Si₃N₄, Sn_(x)Zn_(y)O_(z),TiZnO_(x) or Si_(x)Ti_(y)O_(z), ZnO, ZrO₂, Nb₂O₅ etc. As a (sub)layermaterial with a low refractive index, mention may be made for example ofSiO₂ or possibly a silicon oxycarbide and/or oxynitride, or a mixedoxide of silicon and aluminum or a fluorine-containing compound, forexample of the MgF₂ or AlF₃ type etc.

The stack advantageously comprises for example at least three(sub)layers, the desired reflection being produced by the combinedaction of the various layers of the stack, the layer nearest to thesubstrate being a layer with a high refractive index, the intermediatelayer being a layer with a low refractive index and the outer layerbeing a layer with a high refractive index (for example a stackcomprising the alternately following oxide layers:(substrate)—TiO₂/SiO₂/TiO₂). The (geometric) thickness of eachreflecting layer deposited is generally between 15 and 1000 nm, inparticular between 20 and 1000 nm (the thickness of the substrate beinggenerally a few millimeters, more often around 4 mm). In the case of areflecting layer formed of several (sub)layers (generally in the form ofa stack of layers) the thickness of each of the (sub)layers of the stackmay vary between 5 and 160 nm, generally between 20 and 150 nm (forexample in the case of the TiO₂/SiO₂/TiO₂ stack, it may be of the orderof a few tens of nanometers, for example of the order of 60-80 nm, foreach of the respective TiO₂ and SiO₂ layers, or may be of the order of60-80 nm for TiO₂ layers and of the order of 130-150 nm for the SiO₂layer according to the appearance that is desired for example to be moresilvery or more golden).

The reflecting layer may be applied to the plate, generally afterceramization, in line or in another operation (for example after cuttingand/or forming said plate). It may be applied in particular by pyrolysis(powder, liquid, gaseous), by evaporation or by spraying. Preferably, itis deposited by sputtering and/or by a vacuum deposition method and/or aplasma-assisted method. In particular, the method of depositing a layeror layers by cathode sputtering (for example by magnetron sputtering),in particular assisted by a magnetic field (and with a direct oralternating current) is used, the oxides or nitrides being depositedfrom a suitable metal or alloy or silicon or ceramic etc. target ortargets, if necessary under oxidizing or nitriding conditions(argon/oxygen or argon/nitrogen mixtures as the case may be). Oxidelayers may for example be deposited by reactive spraying of the metal inquestion in the presence of oxygen, and nitride layers in the presenceof nitrogen. In order to make SiO₂ or Si₃N₄ it is possible to start froma silicon target that is lightly doped with a metal such as aluminum soas to make it sufficiently conducting. The (sub)layer or layers chosenaccording to the invention condense on the substrate in a particularlyhomogeneous manner without producing separation or delamination.

In another embodiment of the invention, the reflecting layer may be alayer containing effect pigments (pigments with a metallic effect,nacreous pigments, interference pigments, etc.) as the case may bedissolved in a suitable medium, this layer generally having a spangledeffect. This layer is preferentially based on a molten silicate, frit orglass flux (vitrifiable mixture generally comprising oxides chosen inparticular from those employed in frits for enamel, for example from theoxides of silicon, zinc, sodium, boron, lithium, potassium, calcium,aluminum, magnesium, barium, strontium, antimony, titanium, zirconium,etc.) and the effect pigments used are advantageously formed of aluminumoxide flakes (Al₂O₃) coated with metal oxides. Mention may be made forexample of pigments marketed by Merck under the trade name “Xirallic®”such as TiO₂/Al₂O₃ pigments or interference pigments (Xirallic®T-50-10SW Crystal Silver or Xirallic® T-60-20SW Sunbeam Gold orXirallic® T-60-21 SW Solaris Red or Xirallic® T-60-23SW Galaxy Blue orXirallic® T-60-24SW Stellar Green), or Fe₂O₃/Al₂O₃ pigments (Xirallic®T-60-50SW Fireside Copper or Xirallic® F60-51 Radiant Red).

Other effect pigments that can be used are for example pearl (nacreous)pigments based on mica particles coated with oxides or a combination ofoxides (chosen for example from TiO₂, Fe₂O₃, Cr₂O₃, etc.) such as thosemarketed under the trade name Iriodin® by Merck. Conventional coloringfillers or other pigments may also be incorporated in the aforementionedeffect pigments. The rate of effect pigments may for example be of theorder of 1 to 30% by weight relative to the base (molten silicate, fritor glass flux) in which they are incorporated. The layer may inparticular be deposited by screen-printing (the base and pigments beingas the case may be suspended in a suitable medium generally intended tobe consumed in a subsequent firing step, it being possible in particularfor this medium to include solvents, diluents, oils, resins, etc.), thethickness of the layer being of the order of 1 to 6 μm, for example.

Preferably, as has already been mentioned, the plate according to theinvention is based on a transparent or translucent glass-ceramic and hasa coating on the lower face comprising a background zone formed from apaint having a matt effect as previously defined, and a display zoneformed from a reflecting layer having a mirror effect as previouslydefined or optionally formed from a paint as previously defined that isthinner than the background zone.

The plates according to the invention are generally plates with twosmooth faces without spikes on the lower face.

The plate according to the invention may, as the case may be, beprovided with (or associated with) a functional element or functionalelements or additional decoration(s) (frame, connector(s), cable(s),control element(s), display(s), for example with light-emitting diodesknown as “7-segment diodes”, electronic control strip withtouch-sensitive keys and digital display etc.) and/or may optionallyinclude, apart from the aforementioned contrasted zones, one or moresupplementary, in particular decorative, zones (for example an enamel onthe upper face), but preferentially with limited dimensions.

The coating is generally continuous in each of the zones that can bediscerned on the plate but it is not however excluded to have lesscovering deposits in some places (of the type for example with a spottedpart or with a mesh or with speckles, etc.), the degree of coverage inthese places remaining however preferentially large (close to 100%).

Preferably, the plate according to the invention is intended to beassociated with subjacent induction heating elements. Induction heatinghobs are known and are generally composed of a converter (or generator)connected to a self-induction coil (or inductor) consisting of a windingof conducting wires. The electromagnetic field created by the passage ofa high frequency current produced by the converter generates Foucaultcurrents in the metal base of utensils placed on the plate, enablingthese utensils to be heated rapidly. Variations in temperature and thetemperature maxima to which the plate is subjected are less than thoseobserved with other heating elements, for example halogen or radiantheating elements, and on account of this the plate according to theinvention is particularly well suited to this heating method, thermalshocks to which the coating is subjected being less severe and lesslikely to damage it in the long term.

The plates according to the invention satisfy the requirements in termsof optical properties and safety and have good thermal shock resistanceetc. The plate according to the invention may advantageously be mountedon the insulating support, inside which the heating element or elements,such as inductors, are positioned, without an intermediate complex aimedat masking the inside of the apparatus from the view of the user.

The invention also relates to apparatuses (or devices) for cookingand/or for maintaining a high temperature, comprising at least one plateaccording to the invention (for example cookers and built-in cookingplates). The invention encompasses cooking apparatuses having a singleplate as well as apparatuses having several plates, each of these platesbeing with a single or multiple heater(s). The term “heater” isunderstood to mean a cooking location. The invention also relates tomixed cooked apparatuses of which the cooking plate or platescomprise(s) several types of heaters.

Moreover, the invention is not limited to the production of cookingplates for cookers or cooking surfaces, even though the problem on whichthe invention is based is defined in relation to this application.Plates produced according to the invention may also be other plates thathave to exhibit great insensitivity to temperature variations.

The object of the invention is also a method for producing a plateaccording to the invention.

The production of glass-ceramic plates generally proceeds as follows: ina melting furnace, the glass of the composition chosen to form thisglass-ceramic is melted and then the molten glass is laminated into astandard ribbon or sheet by passing the molten glass between laminatingrollers, and the glass strip is cut up into the desired dimensions. Theplates cut up in this way are then ceramized in a known manner,ceramization consisting of firing the plates following the heatingprofile chosen to convert the glass into the polycrystalline materialcalled “glass-ceramic” of which the coefficient of expansion is zero ornearly zero and which withstands a thermal shock that can extend up to700° C. Ceramization generally comprises a step in which the temperatureis raised progressively to the nucleation region, generally situatedclose to the transformation region of the glass, a step passing throughthe nucleation interval in several minutes, a new progressive increasein temperature up to the temperature of the ceramization stage (orplateau), maintenance of the temperature of the ceramization stage overseveral minutes and then rapid cooling to ambient temperature.

The method generally includes a cutting operation, for example by waterjet, possibly followed by a shaping operation (grinding, beveling,etc.).

The method for producing the plate according to the invention ischaracterized in that at least one first layer (or assembly of(sub)layers) intended to form at least one first zone is applied to oneface of the bare glass-ceramic plate (preferably the face intended to bethe lower face in the position of use) and in which one or more savingsor reserves (uncoated areas) are provided or made, and said reserves arecovered by at least one second layer (or assembly of (sub)layers)intended to form at least one second zone contrasted with respect to thefirst zone or zones.

The layers are applied by any means known to a person skilled in the artaccording in particular to the composition of said layers to be formed.Examples of different possible application methods have been previouslygiven. According to the type of layer, deposition may be carried outbefore ceramization (the case for example of a layer of enamel that canbe baked during ceramization) or after ceramization (the preferred caseof paint and layers of the metallic type or those based on a dielectricmaterial previously described) in line or in a separate operation (forexample after cutting and/or forming said plate).

For the formation of reserves, different means may be used (according tothe type of layer deposited and the precision of the deposition means).The reserves may be left by the deposition technique (screen-printing)or by providing suitable masks, for example made of sheet metal etc. Itis also possible to consider depositing a layer that can be eliminated,in the region of the reserves, by washing, ablation, polishing etc., forexample deposition of a paint that is only hardened or crosslinked inthe desired parts, the remainder being eliminated by washing, ordeposition of a layer or layers of the metallic type or those based on adielectric material in which the reserves are removed by means such aslaser ablation or polishing.

Concerning deposition of the second layer, it can be carried out only inthe reserves (with suitable deposition means or masks for example), oron a larger part of the plate, in particular over all the plate.

Thus, according to a preferred embodiment of the method according to theinvention, there is applied, for example by screen-printing (in one orseveral passes or sub-layers, and while providing reserves) a firstpaint layer, in particular of (matt) paint as previously mentioned, andthen, after heat treating or baking the paint and possible washing ofthe plate, the second layer (or assembly of (sub)layers) is appliedwhile covering all the zones already covered and the reserves by atleast one reflecting layer with a mirror effect, by cathode sputtering(in the case of magnetron sputtering, it is in point of fact easier todeposit the whole plate).

In another embodiment, the deposition order may be reversed, for exampleby depositing at least one continuous reflecting layer on the lower faceof the plate provided with masks in the locations of the chosenreserves, by removing the masks and if necessary covering again theparts already covered by other masks, and then by applying the paint,the paint advantageously only covering, on the final plate obtained,zones not covered by the reflecting layer. The subject of the presentinvention is also an apparatus or device for producing a glass-ceramicplate such as defined above, comprising at least one device for coatingby screen-printing and at least one cathode sputtering device.

The following example illustrates the present invention without howeverlimiting its scope, together with the appended FIGURE showing a view(partial view derived from a photograph) from above in perspective of aplate according to the invention produced in accordance with thisexample.

A transparent glass-ceramic plate 1 is produced, of which the two facesare smooth, from a glass having for example the composition described inapplication EP0437228.

This glass is melted at around 1600° C. in a quantity such that a glassstrip can be laminated, strip from which glass plates are cut having thefinal dimensions 56.5 cm×56.5×0.4 cm.

These glass plates are ceramized on ceramic grids following aceramization cycle comprising the following steps:

a) raising the temperature at 30-80 degrees/minute up to the nucleationregion, generally situated close to the glass transformation region;b) crossing the nucleation interval (670-800° C.) in around 20 minuteswith the temperature being held for a few minutes;c) raising the temperature in 15 to 30 minutes to the temperature T ofthe ceramization plateau of the order of 930° C.;d) holding the temperature T of the ceramization plateau for a time t ofthe order of 20 minutes;e) rapidly cooling to ambient temperature.

The plate obtained is coated (for example by screen-printing, withdilution, where appropriate, of the composition applied in a suitablemedium) with a paint based on at least one polysiloxane resin (obtainedfor example by mixing a silicone polymer and a silicone solution) andfillers (such as a mixture of dark colored pigments, for example so asto give a gray color) over the whole lower face, with the exception:

of locations or reserves consisting of four circular crowns 2 adelimiting the locations which will be situated above the four inductionheating hobs in the position in which the plates are used,

regions 2 b forming symbols (+, − and the operating symbol) as well asseven rectangles delimiting zones which will be situated above thedisplays or indicators (of duration, power etc.),

and essentially decorative signs 2 c (here the first 6 letters of theword “INDUCTION” and a rectangle making the last three letters ION standout in negative).

The paint is then baked or cured in an oven or tunnel. The thickness ofthe layer obtained is for example between 15 and 100 microns. It is thenpossible to see by transparency through the plate, a background zone,for example of a matt gray color, formed of several small zones 3 a, 3b, 3 c, 3 d delimited by the reserves.

Following any possible washing of the plate coated with paint, areflecting layer is deposited, for example one based on Ni—Cr—Fe (inrespective proportions for example of the order of 68%/22%/10%) inthicknesses of the order of 22.5 nanometers (enabling a good compromiseto be obtained for this layer for display zones between hiding theextinguished displays and of seeing them distinctly when they areilluminated) by magnetron sputtering, over the whole surface area of thelower face of the plate already coated with paint. The mirror effectproduced by this very fine reflecting layer is only exerted at thelocation of the reserves provided in the paint layer and does not have anegative effect on the opaque matt gray appearance of the backgroundzone of the glass-ceramic, visible by transparency from above the plate.

A glass-ceramic plate is thus obtained with a pleasing and originalesthetic appearance, this plate having for example a matt and opaquegray/metallized gray background zone (formed by all the zones 3 a, 3 b,3 c, 3 d) hiding the elements of the subjacent structure of the cookingdevice, and including a display zone (formed by all the zones 2 a, 2 b,2 c) with designs having a shiny appearance (for example with a rathersilvery mirror appearance) also hiding the subjacent elements at restwhile enabling them to be seen when they are in the operating state.Since the coating is totally on the lower face, there are no problemswith a double image or problems of the abrasion of the decoration (forexample with rubbing by a saucepan). The plate can be mounted in a frame4, set into a cooking device, etc.

Plates according to the invention can in particular be advantageouslyused for producing a novel range of cooking plates for cookers orcooking surfaces.

1. A glass-ceramic plate serving as a cooking plate for covering orreceiving at least one heating element said plate having a coatingcomprising at least two contrasted zones.
 2. The glass-ceramic plate asclaimed in claim 1, wherein the plate is based on a transparent ortranslucent glass-ceramic and the coating is intended to mask, at leastpartly, the subjacent elements at rest, while enabling the heatingelements and any displays to be detected when they are in service. 3.The glass-ceramic plate as claimed in claim 1, wherein one of the zonesconstitutes a background zone, occupying 50 to 99% of the surface areaof the coating and is intended to mask the main part of the heatingstructure that the plate must cover, and the other zone beingpreferentially a display zone, for revealing, in relation to thebackground, a decoration and/or information and/or the location offunctional elements, while also making it possible to detect whether ornot these elements are in an operational state, the surface area coveredby the latter zone representing 1 to 50% of the surface area of thecoating.
 4. The glass-ceramic plate as claimed in claim 1, wherein thecoating with the contrasted zones is situated on only one and the sameface of the plate, which is the face intended to be turned towards theheating element or elements in the position of use.
 5. The glass-ceramicplate as claimed in claim 1, wherein the two contrasted zones are atleast one matt zone and at least one reflecting zone.
 6. Theglass-ceramic plate as claimed in claim 1, wherein the two contrastedzones are coating zones with variable thicknesses.
 7. The glass-ceramicplate as claimed in claim 1, wherein the coating is formed of at leasttwo layers deposited to form the zones, one of the layers being a firstpaint layer, said layer covering a background zone, the other layerbeing a reflecting layer or a second paint layer thinner than the firstlayer, and covering at least a display zone.
 8. The glass-ceramic plateas claimed in claim 7, wherein the paint has a degradation temperaturegreater than 350° C., and is based on at least one silicone resin and/orat least one high temperature resistant (co)polymer chosen in particularfrom polyimide, polyamide, polyfluorinated and/or polysiloxane resins,the paint optionally contains at least one mineral filler.
 9. Theglass-ceramic plate as claimed in claim 7, wherein the reflecting layeris a layer with a mirror or shiny effect, the reflecting layer beingformed of at least one (sub)layer of the metallic type and/or at leastone (sub)layer based on a dielectric material, or being formed of amolten silicate or a frit or a glass flux also including effectpigments.
 10. The glass-ceramic plate as claimed in claim 1, wherein theplate is associated with the subjacent induction heating elements.
 11. Amethod for producing a plate as claimed in claim 1, wherein at least onefirst layer forming at least one first zone, is applied to one face ofthe bare glass-ceramic plate, and wherein one or more reserves areprovided or made, and said reserves are covered by at least one secondlayer forming at least one second zone contrasted with respect to thefirst zone or zones.
 12. A device for cooking and/or for maintaining ahigh temperature comprising a glass-ceramic plate as claimed in claim 1and one or more heating elements.
 13. A device for producing aglass-ceramic plate as claimed in claim 1, comprising at least onedevice for coating by screen-printing and at least one cathodesputtering device.